Ischemia and hypoxia are common in several pathological conditions, such as cancer, stroke, acute renal failure, and myocardial infarction. Myocardial infarction occurs when the blood supply from a coronary artery is occluded leading to reduced supply (e.g., ischemia) that results in hypoxia to portions of the myocardium. This blockage of the blood flow and oxygen supply is primarily due to an atherosclerosis plaque leading to partial or complete occlusion of the vessel and subsequent myocardial ischemia. Total occlusion of the vessel for more than 4-6 hours results in irreversible myocardial necrosis, but reperfusion within this period can salvage the myocardium and reduce morbidity and mortality. Following an ischemic attack, a series of response mechanisms in the heart, neural, hormonal systems, and vasculature are activated which though initially for beneficial purposes can contribute to worsening of the symptoms and eventual death.
Most of the models of in vivo myocardial ischemia use rodents. In the most commonly used experimental model, the left anterior descending coronary artery commonly called as LADA is ligated. This causes reduction in blood flow and subsequent ischemia. Fluorescent imaging is then used to visualize the vessels. For example, CD31 staining is used to visualize anatomic vessels and DiOC7 is used to visualize perfusion. Finally tissue hypoxia is quantified with EF5, a nitroheterocyclic compound that has been shown to form adducts at a much higher rate in hypoxic tissue. Even though the animal experiments provide detailed representation following ischemia, these experiments are expensive, technically complex and need to overcome ethical concerns.
In this regard, in vitro models were developed to study the effect of ischemia on cultured cardiomyocytes. These models range from treating cells to oxygen deprived media, elevated carbon dioxide levels, reduced nutrient media (absence of serum, etc.), and finally waste accumulation. Common methods rely on altering the cellular metabolism with a chemical agent (e.g., cyanide, azide, Antimycin A, etc.) or altering the external cellular environment by changing gas compositions which is achieved by changes in temperature and rate of change of fresh media. Currently, ischemia studies on myocytes cultured in vitro use one of the following two methods. In the first method, myocytes are cultured in a 35 mm Petri dish. When the cells are nearing confluency, a round glass coverslip is placed over part of the myocyte monolayer surface to restrict nutrient supply and gas diffusion. This rapidly decreases intracellular pH and produces three distinct zones within the monolayer called the ischemic zone (e.g., where the cells don't have access to fresh media and waste metabolites are present in excess), the border zone (e.g., partial cells experience excess waste metabolites and hypoxia whereas the other half of cells are viable), and finally the non-ischemic zone, where the cells have abundant supply of fresh nutrients. A less popular method called the picochamber system can be used to study ischemic conditions on a single cell where the cellular microenvironment oxygen is altered by an argon stream parallel to the surface.
However, all of these models have limitations. Therefore, it would be advantageous to have devices and methods that provide for improved experimental analysis and studies on cells and cultures under ischemic and hypoxic conditions.